This invention relates generally to earth boring percussion drills. More particularly, the invention relates to retention systems for retaining the drill bit of the percussion drill to a guide device.
Conventional percussion drills utilize a variety of drill bits. Some conventional percussion drills utilize a unitary drill bit that is impacted by the hammer. Carbide impact buttons are generally mounted to the work face of the drill bit to provide a cutting surface that has superior hardness and wear resistance. Eventually, wear and damage to the drill bit work face and the impact buttons reduces the efficiency of the drill bit, requiring either replacement or repair of the drill bit.
The method/apparatus that is used to mount the impact buttons to the work face has a large impact on the cost of repairing the drill bit. Mounting methods/apparatus that are designed to facilitate replacement of worn/broken impact buttons reduce the cost of repairing the drill bit. However, such mounting methods/apparatus are generally more prone to failure than mounting methods/apparatus that are designed to "permanently" mount the impact buttons to the work face. Consequently drill bits employing mounting methods/apparatus that are designed to facilitate replacement of the impact buttons generally require repair at more frequent intervals than other drill bits. Although the methods/apparatus for "permanently" mounting impact buttons are less prone to failure, the impact buttons are much more difficult replace.
Some conventional percussive drills utilize a bit impact head that is removably mounted to a guide device. These drills represent a compromise between removably mounting the impact buttons and permanently mounting the impact buttons. The impact buttons for such drills are generally permanently mounted to the bit impact head to reduce the probability of button failure. The bit impact head may be removed from the guide device and sent to a repair facility while a replacement bit impact head is utilized to minimize down-time. Such conventional drill bits often employ a single pin to retain the removable bit impact head to the guide device. Typically, the pin is inserted through a through bore in the guide device and is received within an undercut in the impact head. Since the guide device and the portion of the impact head that is retained within the guide device are generally cylindrical, the pin typically engages the undercut of the impact head at a tangent point. Consequently, such drill bits are frequently subject to failure due to the concentration of stress resulting from such a connection.